Hybrid vehicle and associated engine speed control method

ABSTRACT

A hybrid vehicle and method of control are disclosed wherein the ratio of engine speed to vehicle speed varies continuously in some operating conditions and is controlled to be one of a finite number of preselected ratios in other operating conditions. The transition from continuously variable mode to discrete ratio mode includes selecting a virtual gear number and increasing the engine speed to the corresponding ratio to vehicle speed. Once in a discrete ratio mode, the virtual gear number can vary automatically in response to changes in vehicle speed or in response to driver activation of shift selectors.

TECHNICAL FIELD

This disclosure relates generally to controlling the engine speed andcombined output torque of a hybrid vehicle in response to driver inputs.

BACKGROUND

In a vehicle having a discrete ratio transmission, the speed of thetransmission input shaft is constrained to be proportional to thevehicle speed with a finite set of ratios, except during the briefinterval while the transmission is shifting from one ratio to anotherratio. When the torque converter is locked, the engine speed is alsoconstrained to be proportional to vehicle speed. In a hybrid electricvehicle having a power-split architecture, on the other hand, thetransmission does not mechanically impose a strict relationship betweenthe engine speed and the vehicle speed.

Even in vehicles with automatic transmissions, in which selection of thegear ratio or engine speed is ordinarily determined by a controller,some drivers prefer to occasionally over-ride the controller to provideoperation similar to a manual transmission. Some vehicles are equippedwith shift paddles or other driver interface features which permit thedriver to signal a desire for a higher or a lower gear ratio relative tothe gear ratio automatically selected by the vehicle controller, with anassociated change in engine speed and vehicle torque. In a discreteratio transmission, the controller responds to such a command byshifting to a different one of the discrete gear ratios, which adjustsengine speed accordingly and provides associated torque multiplicationat the vehicle wheels. However, in a vehicle with a continuouslyvariable transmission or similar gearbox, such as a power-split hybrid,the response is more complicated because the transmission does notinherently provide discrete gear ratios with associated different torquemultiplication.

SUMMARY

In various embodiments, a hybrid vehicle control strategy implementsfour different operating modes. The vehicle controller determines whichoperating mode is utilized at any given time in response to operation ofvarious driver interface elements including a shift lever, a downshiftselector, and an upshift selector, for example. In two of the operatingmodes, the controller simulates the operation of a discrete ratiotransmission, both with regard to the engine speed and with regard tothe combined output torque of the engine and one or more tractionmotors. The controller can utilize different logic for shutting theengine off and driving solely with electric power depending on whichoperating mode is active.

In one embodiment, a method of controlling a hybrid vehicle includescontrolling an engine and traction motor to operate in a continuouslyvariable mode and increasing engine speed, in response to operation of adownshift selector, to transition to a discrete ratio mode where theengine and traction motor are controlled to substantially maintain oneof a finite number of preselected virtual gear ratios independent ofaccelerator pedal position. The method can also include, in response todriver operation of an upshift selector while operating in the discreteratio mode, decreasing the engine speed such that a ratio of enginespeed to vehicle speed is substantially equal to a lower one of thefinite number of preselected virtual gear ratios. Further, the methodcan include, in response to a change in vehicle speed while operating inthe discrete ratio mode, adjusting the engine speed such that a ratio ofengine speed to vehicle speed is substantially equal to a different oneof the finite number of preselected virtual gear ratios.

Embodiments according to the present disclosure can also include acontroller for a hybrid electric vehicle having input communicationchannels that receive a vehicle speed, a position of a driver operatedaccelerator pedal, and signals indicating operation of a downshiftselector and an upshift selector; output communication channelsconfigured to control an engine and at least one traction motor; andcontrol logic configured to control engine speed in a continuouslyvariable mode and transition from the continuously variable mode to adiscrete ratio mode in response to operation of the downshift selectorby increasing the engine speed such that the engine speed issubstantially equal to a selected function of vehicle speed from among afinite number of predefined functions of vehicle speed independent ofaccelerator pedal position. In some embodiments, each of the predefinedfunctions of vehicle speed corresponds to a linear or proportionalrelationship between engine speed and vehicle speed over a range ofvehicle speeds. In some embodiments, the controller also includescontrol logic configured to respond to changes in vehicle speed, whileoperating in the discrete ratio mode, by adjusting the engine speed suchthat the engine speed is substantially equal to a different one of thepredefined functions. In some embodiments, the controller also includescontrol logic configured to respond to operation of the downshift orupshift selector, while operating in the discrete ratio mode, byincreasing or decreasing, respectively, the engine speed such that theengine speed is substantially equal to a different predefined functionfrom among the finite number of predefined functions.

Embodiments according to the present disclosure can also include avehicle having a transmission with a planetary gear set wherein elementsof the planetary gear set are driveably connected to an engine a firsttraction motor and a set of wheels, a second traction motor driveablyconnected to the set of wheels, and a controller in communication withthe engine, the traction motors, an accelerator pedal, an upshiftselector, and a downshift selector. In such embodiments, the controllercan be programmed to operate the engine in a continuously variable mode,operate the engine in a discrete ratio mode, and transition from thecontinuously variable mode to the discrete ratio mode in response tooperation of the downshift selector, the transition including increasingthe engine speed such that the engine speed is substantially equal to aselected function of vehicle speed from among the finite number ofpredefined functions of vehicle speed. In some embodiments, each of thepredefined functions of vehicle speed corresponds to a linear orproportional relationship between engine speed and vehicle speed over arange of vehicle speeds. In some embodiments, the controller is furtherprogrammed to respond to changes in vehicle speed, while operating inthe discrete ratio mode, by adjusting the engine speed such that theengine speed is substantially equal to a different one of the finitenumber of predefined functions of vehicle speed. In some embodiments,the controller is further programmed to respond to operation of thedownshift or upshift selectors, while operating in the discrete ratiomode, by increasing or decreasing, respectively, the engine speed suchthat the engine speed is substantially equal to a different one of thefinite number of predefined functions of vehicle speed.

Various embodiments according to the present disclosure can provide oneor more advantages. For example, systems and methods for controlling ahybrid vehicle according to the present disclosure mimic or emulate amanual or select shift mode of an automatic step-ratio transmission in ahybrid vehicle having a continuously variable transmission or similargearbox. In addition, various strategies of the present disclosureprovide drivers of hybrid vehicles more interactive controls to manuallycommand powertrain speed and acceleration to provide enhanced luxuryfeatures and a sporty feel.

The above advantages and other advantages and features will be readilyapparent from the following detailed description of the preferredembodiments when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a vehicle powertrain,controller, and user interface features of a representative embodimentof a hybrid vehicle according to the present disclosure;

FIG. 2 is a state transition chart illustrating operation of a system ormethod of an embodiment of the present disclosure;

FIG. 3 is a flow chart illustrating operation of a system or methodaccording to various embodiments when in a Normal operating mode;

FIG. 4 is a graph illustrating a relationship between vehicle speed,accelerator pedal position, and wheel torque command of a representativeembodiment according to the disclosure;

FIG. 5 is a flow chart illustrating operation of a system or methodaccording to various embodiments when in the Live-In-Drive (LID)operating mode;

FIG. 6 is a graph illustrating a relationship between vehicle speed,virtual gear number, and engine speed of a representative embodimentaccording to the disclosure;

FIG. 7 is a graph illustrating a relationship between actual acceleratorpedal position, virtual gear number or operating mode, and modifiedpedal position of a representative embodiment according to thedisclosure;

FIG. 8 is flow chart illustrating the selection of an initial virtualgear number when transitioning into LID or Select Shift Transmission(SST) operating modes of one embodiment of the disclosure;

FIG. 9 is flow chart illustrating operation of a system or methodaccording to embodiments of the disclosure when in the Sport operatingmode;

FIG. 10 is a flow chart illustrating operation of a strategy forshutting off and restarting the engine in certain operating modes ofvarious embodiments of the disclosure; and

FIG. 11 is a flow chart illustrating operation of a system or methodwhen in the SST operating mode according to various embodiments of thedisclosure.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

A powertrain for a hybrid electric vehicle is illustrated schematicallyin FIG. 1. The powertrain includes an internal combustion engine 20driveably connected to a planet carrier 22, a generator 24 driveablyconnected to a sun gear 26, and an output shaft 28 driveably connectedto a ring gear 30. Elements are driveably connected when there is amechanical power flow path between them such that the speeds of theelements are constrained to be substantially proportional. Planetcarrier 22 supports a set of planet gears 32 such that each planet gearis in continuous meshing engagement with sun gear 26 and ring gear 30.Output shaft 28 drives the vehicle wheels directly or indirectly, suchas via a differential assembly, for example.

Traction motor 34 is driveably connected to the output shaft 28. Boththe generator 24 and the traction motor 34 are reversible electricalmachines that are capable of converting electrical power into rotationalmechanical power or converting rotational mechanical power intoelectrical power. The terms generator and motor should be regardedmerely as labels for ease of description and does not limit the functionor operation of either electrical machine. Generator 24 and tractionmotor 34 are both electrically connected to battery 36.

The rotational speed of sun gear 26, carrier 22, and ring gear 30 arelinearly related such that speed of carrier 22 is a weighted average ofthe speed of sun gear 26 and ring gear 30. Consequently, the speed ofthe engine 20 is not constrained to be proportional to the speed of theoutput shaft 28 in this arrangement. Instead, the engine speed can beselected or controlled independently of the vehicle speed by setting thegenerator speed accordingly. Power flows from the engine to the outputshaft through a combination of mechanical power transfer and electricalpower transfer. During some operating conditions, the engine 20 cangenerate more power than what is delivered to the output shaft 28 withthe difference, neglecting efficiency losses, delivered to battery 36.Under other operating conditions, the battery 36 in combination withgenerator 24 and/or traction motor 34 can supplement the power deliveredby the engine 20 such that more power is delivered to the output shaft28.

The engine 20, generator 24, and traction motor 34, all respond tocontrol signals from controller 38. These control signals determine theamount of torque generated. The controller also receives speed signalsfrom the engine 20, generator 24, and traction motor 34 and a state ofcharge signal from battery 36. The controller accepts input signalsindicating driver intention from a brake pedal 40, an accelerator pedal42, a shift lever 44, a steering wheel 46, a downshift selector 48, anupshift selector 50, and a cruise control button 51. Shift lever 44allows the driver to select Park, Reverse, Neutral, Drive, and Sportdriving modes.

The top level control states are illustrated in FIG. 2. The controllerstarts in state 60 and transitions to Normal mode 62 as soon as thedriver selects the Drive (D) position using shift lever 44. Operation inNormal mode is illustrated by the flow diagram of FIG. 3. In Normalmode, the controller repeatedly performs the operations of setting theoutput torque 66, setting the engine mode 68, and setting the enginespeed 70. In Normal mode, the target output torque is calculated at step66 based on accelerator pedal position and vehicle speed using a tablesuch as that illustrated in FIG. 4. Vehicle speed can be calculated fromtraction motor speed or wheel speed sensors. Engine mode is set toeither running or stopped at step 68 using a variety of input signalsincluding battery state of charge, output power command, acceleratorpedal position, and vehicle speed. If the engine mode is running, atarget engine speed is calculated to minimize fuel consumption whiledelivering the desired output torque and maintaining the battery at adesired state of charge. In this Continuously Variable Transmission(CVT) mode, the engine speed varies continuously, as opposed to varyingin discrete steps, in response to changes in accelerator pedal positionand vehicle speed. Finally, operating parameters of the engine,generator, and traction motor are adjusted such that the actual outputtorque and engine speed tend toward the selected targets.

Referring again to FIG. 2, the controller transitions from Normal mode62 to Live-In-Drive (LID) mode 72 whenever the driver activates thedownshift selector 48. LID mode simulates the experience of driving avehicle with a discrete ratio transmission. Operation in LID mode isillustrated by the flow diagram of FIG. 5. Upon entering LID mode, thecontroller selects an initial virtual gear ratio at step 74 and thenrepeatedly performs the operations of setting the output torque at steps76 and 66′, setting the engine speed at step 78, and updating thevirtual gear ratio in steps 80 and 82. Each of these operations isdiscussed in additional detail below. As shown in FIG. 2, a number ofconditions cause the controller to transition back to Normal mode 62including vehicle speed dropping below a low threshold value or anautomatically selected downshift. Additionally, a transition can betriggered when the controller detects a cruising condition, as indicatedby activation of the cruise control 51, or a tip-out condition,indicated by a reduction in accelerator pedal position, and thecondition persists for some predetermined amount of time. This lattertype of condition will not result in a transition, however, if thecontroller detects a high driver workload at step 84, such as might beindicated by large displacements of steering wheel 46, large yaw, pitch,or roll rates, or high longitudinal or lateral accelerations, forexample.

As also shown in FIG. 5, in LID mode 72, the engine speed is calculatedin step 78 based on the vehicle speed and the virtual gear number asillustrated in FIG. 6. For a particular virtual gear number (1^(st)through 8^(th) in the representative embodiment illustrated), the enginespeed (We) is directly proportional to vehicle speed (V), as it would bewith a step ratio transmission. However, if that fixed ratio wouldresult in an engine speed (We) less than a minimum engine speed(We_(min)), the engine speed (We) is set to the minimum engine speed(We_(min)). Similarly, the engine speed (We) is not set higher than amaximum engine speed (We_(max)). Both minimum and maximum engine speedscan be a function of vehicle speed (V).

In step 76, a modified accelerator pedal position is calculated from themeasured accelerator pedal position using a table such as illustrated inFIG. 7. This modified accelerator pedal position is used in place of theactual pedal position in step 66′ to calculate the target output torque.The curves in FIG. 7 are selected to simulate the output torquecapability of a powertrain with a discrete ratio transmission.Specifically, as the virtual gear number (1^(st) through 8^(th) in thisexample) increases, the resulting target output torque is lower for anygiven non-zero accelerator pedal position. The combined effect of steps76 and 66′ is operation of the engine and at least one traction motorsuch that combined output torque corresponds to one of a plurality ofoutput torque functions, each output torque function having a distinctoutput torque at a maximum value of accelerator pedal position for anassociated vehicle speed.

As also shown in FIG. 5, in step 80, the controller checks foractivations of either the upshift selector or the downshift selector andadjusts the virtual gear number accordingly. In step 82, the controllerdetermines if there is a need to automatically adjust the virtual gearnumber. In particular, an upshift can be triggered by an increase invehicle speed. Similarly, a downshift can be indicated when vehiclespeed decreases. However, as mentioned previously, the controllertransitions back to Normal mode 62 when an automatic downshift isindicated. The automatic shift criteria are calibrated such thatautomatic changes in virtual gear number are less common than shifts ina traditional discrete ratio automatic transmission.

The algorithms for calculating target engine speed and target outputtorque both utilize the virtual gear number. Therefore, an initialvirtual gear number is determined upon transitioning into LID mode. Atstep 74, the controller selects an initial virtual gear number that willresult in an increase in engine speed. The procedure for setting theinitial virtual gear number is further illustrated in the flow chart ofFIG. 8. In step 84, the controller calculates We_(max), the maximumengine speed at the current vehicle speed, using a formula or a lookuptable, for example. Next, in step 86, the controller computesGear_(min), the lowest virtual gear number for which the target enginespeed would be less than We_(max) at the current vehicle speed. Thisstep can be done with either an iterative algorithm or using a lookuptable. Next, the controller computes the target engine speedcorresponding to Gear_(min) at the current vehicle speed, W(V,Gear_(min)). In step 88, this is compared to the current engine speed,We_(current). If We_(current) is greater than W(V, Gear_(min)), then thetarget engine speed will be restricted by maximum engine speed.Consequently, in step 90, the target gear is set to Gear_(min)−1 and thetarget engine speed is set to We_(max). However, in the more typicalcase where We_(current) is less than W(V, Gear_(min)), step 92 selectsthe highest virtual gear number that will result in an increase inengine speed relative to the current engine speed.

Referring once again to FIG. 2, the controller transitions from Normalmode 62 to Sport mode 94 whenever the driver moves the shift lever 44 tothe Sport (S) position. Operation in Sport mode is illustrated by theflow diagram of FIG. 9. The controller repeatedly performs theoperations of setting the output torque 96 and 66″, setting the enginespeed 99, and setting the engine mode 98. To provide a more sportyreaction to accelerator pedal movements, the target output torque iscomputed based on a modified accelerator pedal position as illustratedby the upper heavy line 238 in FIG. 7. The mapping between actualaccelerator pedal position and modified accelerator pedal position isselected such that the value is equal at the minimum 237 and maximum 239values, but the modified value is higher for all intermediate levels.

As also shown in FIG. 9, target engine speed is set in step 99 using asimilar algorithm to that used in Normal mode. However, the targetengine speed is scaled up by a designated amount, such as 10-20% forexample, relative to the value that would be used in Normal mode. Unlikethe algorithm for setting engine mode used in Normal mode, the algorithmused in Sport mode as indicated at step 98 only stops the engine whenthe vehicle is stationary and the brake pedal is depressed. The modifiedengine mode setting algorithm is illustrated in FIG. 10. If the engineis currently stopped 100, then the engine is restarted at step 102 ifthe vehicle is moving 104 or the brake pedal is released 106. Similarly,if the engine is currently running, then the engine is stopped at step108 only if the vehicle is stationary 110 and the brake pedal is pressed112.

If the driver activates either the upshift of downshift selector whilein Sport mode 94, the controller transitions to Select ShiftTransmission (SST) mode 114, as shown in FIG. 2. In SST mode, the targetengine torque and target engine speed are set to simulate a discreteratio transmission, as described with respect to LID mode. However, thecontroller will remain in SST mode until the driver indicates a desireto leave this mode by either holding a shift selector 48 or 50 forseveral seconds of by moving shift lever 44 back to the Drive (D)position. Operation in SST mode is illustrated by the flow diagram ofFIG. 11. In SST mode, the virtual gear number is adjusted at step 80′ inresponse to activation of downshift selector 48 and upshift selector 50in the same manner as in LID mode. In addition, the controller canautomatically adjust the virtual gear number, either up or down, inresponse to changes in vehicle speed or accelerator pedal position. Thisautomatic feature sets the virtual gear number to 1st gear as thevehicle comes to a stop. However, the driver can override this selectionby manipulating the shift selectors while the vehicle is stationary instep 118. In SST mode, the engine mode depends on the virtual gearnumber, vehicle speed, and accelerator pedal position. In step 120, thecontroller calculates an engine shutdown limit, which is an acceleratorpedal position below which electric drive is enabled. The shutdown limitis a function of output power demand, virtual gear number, and vehiclespeed. The shutdown limits for several gear ratios at a particularvehicle speed and output power demand are illustrated by black circlesin FIG. 7. When one of the higher virtual gear numbers, i.e. 5th-8th, isactive and the accelerator pedal position is less than the shutdownlimit, the normal engine mode algorithm 68′ of Normal mode is used. If alower virtual gear number, i.e. 1st-4th, is active, or if theaccelerator position is above the engine shutdown limit, then the morerestrictive algorithm 98′ of Sport and LID modes is used.

As illustrated by the representative embodiments described above,various embodiments according to the present disclosure can provide oneor more advantages, such as emulating a manual or select shift mode ofan automatic step-ratio transmission in a hybrid vehicle having acontinuously variable transmission or similar gearbox. In addition,various strategies of the present disclosure provide drivers of hybridvehicles more interactive controls to manually command powertrain speedand acceleration to provide enhanced luxury features and a sporty feel.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention. While variousembodiments may have been described as providing advantages or beingpreferred over other embodiments with respect to one or more desiredcharacteristics, as one skilled in the art is aware, one or morecharacteristics may be compromised to achieve desired system attributes,which depend on the specific application and implementation. Theseattributes include, but are not limited to: cost, strength, durability,life cycle cost, marketability, appearance, packaging, size,serviceability, weight, manufacturability, ease of assembly, etc. Theembodiments described herein that are described as less desirable thanother embodiments or prior art implementations with respect to one ormore characteristics are not outside the scope of the disclosure and maybe desirable for particular applications.

What is claimed is:
 1. A method of controlling a hybrid vehiclecomprising: controlling an engine and traction motor to operate in acontinuously variable mode; and increasing engine speed to transition toa discrete ratio mode where the engine and traction motor are controlledto substantially maintain one of a finite number of preselected virtualgear ratios independent of accelerator pedal position.
 2. The method ofclaim 1 wherein the transition is initiated in response to operation ofa downshift selector.
 3. The method of claim 1 further comprising inresponse to driver operation of a downshift selector while operating inthe discrete ratio mode, increasing the engine speed such that a ratioof engine speed to vehicle speed is substantially equal to a higher oneof the finite number of preselected virtual gear ratios.
 4. The methodof claim 1 further comprising in response to driver operation of anupshift selector while operating in the discrete ratio mode, decreasingthe engine speed such that a ratio of engine speed to vehicle speed issubstantially equal to a lower one of the finite number of preselectedvirtual gear ratios.
 5. The method of claim 1 further comprising inresponse to a change in vehicle speed while operating in the discreteratio mode, adjusting the engine speed such that a ratio of engine speedto vehicle speed is substantially equal to a different one of the finitenumber of preselected virtual gear ratios.
 6. A controller for a hybridelectric vehicle, the controller comprising: input communicationchannels that receive a vehicle speed, a position of a driver operatedaccelerator pedal, and signals indicating operation of a downshiftselector; output communication channels configured to control an engineand at least one traction motor; and control logic configured to controlengine speed in a continuously variable mode; and transition from thecontinuously variable mode to a discrete ratio mode in response tooperation of the downshift selector by increasing the engine speed suchthat the engine speed is substantially equal to a selected function ofvehicle speed from among a finite number of predefined functions ofvehicle speed independent of accelerator pedal position.
 7. Thecontroller of claim 6 wherein each of the predefined functions ofvehicle speed corresponds to a linear relationship between engine speedand vehicle speed over a range of vehicle speeds.
 8. The controller ofclaim 7 wherein each of the predefined functions of vehicle speedcorresponds to a proportional relationship between engine speed andvehicle speed over a range of vehicle speeds.
 9. The controller of claim6 further comprising control logic configured to respond to changes invehicle speed, while operating in the discrete ratio mode, by adjustingthe engine speed such that the engine speed is substantially equal to adifferent one of the predefined functions.
 10. The controller of claim 6further comprising control logic configured to respond to operation ofthe downshift selector, while operating in the discrete ratio mode, byincreasing the engine speed such that the engine speed is substantiallyequal to a different predefined function from among the finite number ofpredefined functions.
 11. The controller of claim 6 further comprisingan input communication channel that receives signals indicatingoperation of an upshift selector; and control logic configured torespond to operation of the upshift selector, while operating in thediscrete ratio mode, by decreasing the engine speed such that the enginespeed is substantially equal to a different one of the predefinedfunctions.
 12. A vehicle comprising: a transmission, the transmissionhaving a planetary gear set, a first element of the planetary gear setdriveably connected to an engine, a second element of the planetary gearset driveably connected to a first traction motor, a third element ofthe planetary gear set driveably connected to a set of wheels; a secondtraction motor driveably connected to the set of wheels; and acontroller, the controller in communication with the engine, thetraction motors, an accelerator pedal, an upshift selector, and adownshift selector, the controller programmed to operate the engine in acontinuously variable mode, wherein the engine speed varies continuouslyin response to changes in accelerator pedal position for each vehiclespeed; operate the engine in a discrete ratio mode, wherein the enginespeed is maintained substantially equal to one of a finite number ofpredefined functions of vehicle speed independent of accelerator pedalposition; and transition from the continuously variable mode to thediscrete ratio mode in response to operation of the downshift selector,the transition including increasing the engine speed such that theengine speed is substantially equal to a selected function of vehiclespeed from among the finite number of predefined functions of vehiclespeed.
 13. The vehicle of claim 12 wherein each of the predefinedfunctions of vehicle speed corresponds to a proportional relationshipbetween engine speed and vehicle speed over a range of vehicle speeds.14. The vehicle of claim 12 wherein the controller is further programmedto respond to changes in vehicle speed, while operating in the discreteratio mode, by adjusting the engine speed such that the engine speed issubstantially equal to a different one of the finite number ofpredefined functions of vehicle speed.
 15. The vehicle of claim 12wherein the controller is further programmed to respond to operation ofthe downshift selector, while operating in the discrete ratio mode, byincreasing the engine speed such that the engine speed is substantiallyequal to a different one of the finite number of predefined functions ofvehicle speed.
 16. The vehicle of claim 12 wherein the controller isfurther programmed to respond to operation of the upshift selector,while operating in the discrete ratio mode, by decreasing the enginespeed such that the engine speed is substantially equal to a differentone of the finite number of predefined functions of vehicle speed.